The microstrip technique is a planar technique with applications to making signal transmission lines and to making antennas constituting a coupling between such lines and radiated waves. It employs conductive patches and/or strips formed on the top surface of a thin dielectric substrate which separates them from a conductive ground layer on the bottom surface of the substrate. A patch of the above kind is typically wider than a strip of the above kind and its shape and dimensions constitute important characteristics of the antenna. The substrate is typically in the form of a rectangular plane sheet of constant thickness. This is in no way obligatory, however. In particular, it is known that an exponential variation in the thickness of the substrate widens the bandwidth of an antenna of the above kind and that the shape of the sheet can depart from the rectangular shape. The electric field lines extend through the substrate between the strip or the patch and the ground layer. The above technique differs from various other techniques that also use conductive elements on a thin substrate, namely:
the stripline technique in which a strip is confined between the bottom ground layer and a top ground layer which in the case of an antenna must include a slot to enable coupling with the radiated waves, PA1 slotted line techniques in which the electric field is established between two parts of a conductive layer formed on the top surface of the substrate and separated from each other by a slot which in the case of an antenna must typically open into a wider opening facilitating coupling with the radiated waves, for example by forming a resonant structure, and PA1 the coplanar line technique in which the electric field is established on the top surface of the substrate and symmetrically between a central conductive strip and two conductive areas on respective opposite sides of the strip from which they are separated by respective slots. In the case of an antenna, the strip is typically connected to a wider patch to form a resonant structure providing a coupling with the radiated waves. PA1 the configuration of the patches, which can include slots, possibly radiating slots, PA1 the presence and the location of any short-circuits and of electrical models representative of short-circuits, although the latter cannot always be deemed to be equivalent, even approximately, to perfect short-circuits of zero impedance, and PA1 coupling devices included in such antennas for coupling their resonant structures to a signal processing unit such as a transmitter, and the location of such devices. PA1 two parallel dielectric layers each having a bottom surface, a top surface and an edge surface, PA1 a conductive ground plane under the bottom surface of the bottom dielectric layer, PA1 a conductive patch extending between the two dielectric layers and having two ends folded over onto the top face of the top dielectric layer, this antenna being similar to a cavity radiating through two lateral openings, PA1 two short-circuit conductors on the edge surface of the bottom dielectric layer connecting the patch two the ground plane, and PA1 connecting conductors for transmitting a signal between the antenna and a signal processing unit. PA1 facilitating the coupling between a short-circuit antenna of the above kind and a signal processing unit such as a transmitter that has to cooperate with the antenna, and PA1 limiting the cost of manufacture of a communication device including an antenna of the above kind and a signal processing unit, especially in the case of mass production of a device of the above kind. PA1 a dielectric substrate having a bottom surface, a top surface and an edge surface, PA1 a conductive ground plane on said bottom surface, PA1 a conductive patch on said top surface, PA1 two short-circuit conductors on said edge surface and connecting said patch to said conductive ground, and PA1 connecting conductors for transmitting a signal between said antenna and a signal processing unit; wherein the connecting conductors include a coplanar line having a first section on the top face of the substrate and a second section on the edge surface and extending the first section with no significant impedance discontinuity.
With regard to the manufacture of antennas, the following description will on occasion and for simplicity be restricted to the case of a transmit antenna connected to a transmitter. It must nevertheless be understood that the arrangements described could equally apply to receive antennas connected to a receiver. With the same aim of simplicity it will be assumed that the substrate is in the form of a horizontal sheet.
Broadly speaking, a distinction can be made between two fundamental types of resonant structure that can be implemented in microstrip technology. The first type might be called a "half-wave" structure. The antenna is then a "half-wave" or "electric" antenna. Assuming that one dimension of the patch constitutes a length and extends in a longitudinal direction, the length is substantially equal to half the wavelength of an electromagnetic wave propagating in that direction in the line consisted by the ground plane, the substrate, and the patch. Coupling with the radiated waves occurs at the ends of the length, the ends being in regions where the amplitude of the electric field in the substrate is maximal.
A second type of resonant structure that can be implemented using the same technology might be called a "quarter-wave" structure. The antenna is then a "quarter-wave" or "magnetic" antenna. It differs from a half-wave antenna firstly in that its patch has a length substantially equal to one fourth of the wavelength, with the length of the patch and the wavelength being defined as above, and secondly in that there is a hard short-circuit at one end of the length between the ground plane and the patch so as to impose a quarter-wave type resonance with a node of the electric field fixed by the short-circuit. The coupling with the radiated waves occurs at the other end of the length, which is in the region in which the amplitude of the electric field through the substrate is maximal.
In practice various types of resonance can occur in such antennas. They depend in particular on:
For a given antenna configuration there may be more than one resonant mode enabling use of the antenna at a plurality of frequencies corresponding to the resonant modes.
An antenna of the above kind is typically coupled to a signal processing unit such as a transmitter not only by means of a coupling device included in the antenna but also by means of a connecting line external to the antenna and connecting the coupling device to the signal processing unit. Considering an overall functional system including the signal processing unit, the connecting line, the coupling device, and the resonant structure, the coupling device and the connecting line must be made so that the system has a uniform impedance throughout its length, which avoids spurious reflections opposing good coupling.
In the case of a transmit antenna having a resonant structure, the respective functions of the coupling device, of the connecting line, and of the antenna are as follows: the function of the connecting line is to transport a radio frequency or microwave frequency signal from the transmitter to the terminals of the antenna. All along a line of the above kind the signal propagates in the form of a traveling wave without any significant modification of its characteristics, at least in theory. The function of the coupling device is to convert the signal supplied by the connecting line to a form in which it can excite resonance of the antenna, i.e. the energy of the traveling wave carrying the signal must be transferred to a standing wave established in the antenna with characteristics defined by the antenna. As for the antenna, it transfers energy from the standing wave to a wave that is radiated into space. The signal supplied by the transmitter is therefore converted a first time from the form of a traveling wave to that of a standing wave and then a second time to the form of a radiated wave. In the case of a receive antenna the signal takes the same forms in the same units but the conversions are carried out in the opposite direction and in the reverse order.
The connecting lines can be implemented in a non-planar technology, for example in the form of coaxial lines.
Planar technology antennas are used in various types of equipment. They include mobile telephones, base stations for mobile telephones, automobiles, aircraft, and missiles. In the case of a mobile telephone, the continuous nature of the bottom ground layer of the antenna means that the radiated power intercepted by the body of the user of the device is easily limited. In the case of automobiles, and above all in the case of an aircraft or a missile whose outside surface is a metal surface and has a curved profile to minimize drag, the antenna can be conformed to that profile so as not to generate any unwanted additional drag.
The present invention is more particularly concerned with quarter-wave antennas with small dimensions.
A first quarter-wave microstrip antenna is described in the article by T. D. Ormiston, P. Gardner and P. S. Hall "Microstrip Short-Circuit Patch Design Equations", Microwave and Optical Technology Letters, vol. 16, No. 1, September 1997, pages 12-14.
In FIG. 1 of the above article, the substrate and the ground layer of the antenna are not shown, but the presence of a substrate and a ground layer under the patch and the microstrip shown is implied. To impose quarter-wave resonance on the antenna one edge of the patch is provided with a short-circuit formed in a conductive layer on an edge surface of the substrate. The short-circuit is a composite one, i.e. it comprises two conductors in the form of vertical strips. The strips extend laterally to respective ends of the width of the patch with an axial gap between them.
The article describes means for feeding the antenna from a transmitter. They are designated by the term "microstrip", i.e. they employ the microstrip technology. Although it is not explained in the article, it is clear that the microstrip means provide the two above-specified functions of the coupling device and of the connecting line. FIG. 1 of the article shows that the connecting line is a standard microstrip line. A main conductor of the line is a strip shown to be in the plane of the patch. A ground conductor of the line is part of the ground layer, not shown, common to the line, to the coupling device, and to the antenna.
As for the coupling device, it is in the form of a horizontal longitudinal strip. It is shown as part of a microstrip line extending the strip of the connecting line. This strip might be called the coupling strip. It enters the area of the patch via the edge of the short-circuit. It then extends into that area from the edge between two notches and is connected to the patch at a connection point internal to the patch, i.e. at a point inside the area of the patch. According to the article, the two notches are provided to enable the coupling strip to penetrate as far as the appropriate connection point. They correspond to the two edges of the axial gap of the short-circuit.
This first prior art antenna has the following drawbacks:
A first drawback relates to the fact that the strip and the ground of the connecting line are respectively in line with the patch and with the ground of the antenna. At least in some small devices such as some mobile phones, the components of the transmitter are inside the unit including the antenna and the antenna is on the surface of the device, the components typically being grouped together on a printed circuit board called the "mother board". As a result the connecting line described in the above article cannot on its own connect the antenna to the transmitter. An additional connecting line must be provided and installing two such lines in a device of the above kind increases its manufacturing cost.
Another drawback of the above antenna is that it can be fed, or more generally coupled to the signal processing unit, only when various parameters are adjusted precisely. These parameters include the width and the length of the two notches mentioned above and the width of the coupling strip, and they must be adjusted to obtain a suitable value of the impedance of the antenna. Their values, and more particularly the Length, must be kept within very close tolerances that are difficult to determine in advance. In the case of industrial mass production of such antennas, this adjustment problem can increase manufacturing costs unacceptably.
A second quarter-wave microstrip antenna is described in patent document WO 94/24723 (Wireless Access Inc). Its patch (316 in FIG. 3) has a wide slot (rectangular ring 350) to make it less sensitive to the proximity of conductive masses such as a human body or electrical circuits such as those of a microcomputer. Its short-circuit (330) is partial in the sense that it is formed by only a segment of one edge of the patch. It is stated that this facilitates matching the input impedance of the antenna. The connecting line feeding the antenna is disposed vertically under the substrate. It is of the coaxial type. The coupling device is an extension of the central conductor, i.e. of the main conductor that extends along the axis of the line, the extension passing through the substrate in order to be connected to the patch. The ground conductor that sheathes the line is connected directly to the antenna ground.
The second prior art antenna has the drawback that providing an efficient coupling device using the terminal part of the central conductor of a coaxial line connected to the antenna patch requires a hole through the substrate and leads to practical difficulties, in particular with adjusting the position of the connection point. These problems increase the cost of manufacture, especially in the case of mass production.
Patent Application EP 0 795 926 describes an antenna having:
The connecting conductors include a first microstrip waveguide on the top face of the bottom dielectric layer, by virtue of the fact that is formed by a cut-out in the patch. In a first embodiment the first microstrip waveguide is connected to a coaxial cable below the ground plane by a conductive strip very much narrower than the first guide on the edge surface of the bottom dielectric layer.
In a second embodiment the coaxial cable is replaced by a second microstrip waveguide in the ground plane, on the bottom surface of the bottom dielectric layer, if it is designed like a printed circuit board.
The above antenna has the disadvantage of a non-negligible impedance discontinuity at the connection between the first waveguide and the coaxial cable or the second microstrip waveguide.